Deposition of very small water droplets on solid surface, resulting in “fogging”, may cause inconvenience and, in some circumstances, create a safety hazard. Such fogging occurs on vehicular windshields, when vehicles are operated in cold and humid conditions. It also occurs on eye glasses, when a spectacled person enters a warm house from a colder outdoor environment in the winter, or from an air-conditioned building to the warm outdoor environment in the summer. Bathroom mirrors become fogged while someone is taking a hot shower. Moreover fogging of swimming goggles, in a hot food showcase, or in a greenhouse, windows of buildings, trains, ships, spacecraft, night vision goggles, or anywhere else where a viewing surface is involved, such as military sighting/aiming equipment and instruments, compromise safety or impair efficiency. In this respect, an anti-fogging solution, to address these problems, would be embraced by a relatively large market.
Fog generally occurs when a mirror surface crosses areas with a relatively large temperature gradient, resulting in the condensation of vapour to form tiny liquid droplets on the surface. When the surface contact angle of these droplets is higher than 40°, visible light is reflected and refracted, leading to the consequent reduction of transmission and reflection of visible lights (B. J. Briscoe, K. P. Galvin, Sol. Energy 1991, 46, 191). To mitigate this situation, there are two approaches. The first approach is to treat the surface to render such surface more hydrophobic, so that tiny water droplets do not adhere to such surface. Unfortunately, present techniques are relatively uneconomical and do not work particularly well with tiny droplets. The second approach is to decrease the water contact angle on the subject surface. With this approach, ideally, the water droplets completely spread and form a uniform thin film over the surface. Compared with the first approach, this one is widely studied and many methods have been developed including coating the surface with surfactants, hydrophilic polymers and salts with certain photo effect. As a result, the generally accepted anti-fogging strategy is to efficiently increase the surface free energy of substrates so that water can spread over them.
Song describes a windshield anti-fogging agent that mainly contains sodium dodecyl sulphate, sodium alkyl sulfosuccinate, methyl ethyl glycol, isopropanol, dodecanol, ethylene glycol, trihydroxyethyl methyl amonium methosulfate, sodium benzoate, flavouring essence and water (G. Song, 2007, CN 1990815A). This formulation is relatively inexpensive and can be easily applied. In order to extend the working period of anti-fogging coatings, Gao and co-inventors in-situ polymerize hydrophilic polymers with hydroxyl or amino group on the surface of glass. They have also mixed Al2O3, SiO2 and ZrO2 into the final coating to improve the coating's abrasive resistance. However, they also acknowledge that the coating requires to be cured under 100-180° C. for 0.5-5 hours (J. Gao, H. Yang, 2006, CN 1818004A). Aine coated a resin lens with a complex of fatty acid amide and block copolymer of EO-PO (ethylene oxide and propylene oxide) (H. Aine, 2008, CN101161756A). Zhang and Liu applied a self-assembly method to react hydrophilic trialkoxy-3-aminosilane with hydroxyl groups on quartz or optical glass (J. Zhang, X. Liu, 2010, CN 101788693A). This method requires pre-acidulating the substrates to produce hydroxyl groups. Chen coated resin lens with polysiloxane (2 wt %) and water soluble polyurethane (58 wt %) to produce an anti-fogging surface (M. Chen, 2003, EP 1325947A2). Youngblood and co-workers used a layer-by-layer method to graft fluorine based surfactants on the surface of silicon wafer to produce anti-fogging surface, and the water contact angle on the coated surface was about 20° (J. A. Howarter, J. P. Youngblood, Macromol. Rapid Commun., 2008, 29, 455-466). Heberger and co-inventors pre-coated substrates with a polymer base, and then coated fluorine based surfactants and copolyester binders to produce anti-fogging coatings (J. M. Heberger et al, 2001, EP 1110993 A2).
Aside from the above-described methods, another approach to producing an anti-fogging coating is to coat glass surface with special metallic oxides possessing photo-effect, e.g. TiO2. Wanatabe and co-workers studied the wettability transferring of TiO2 under UV irradiation (R. Wang, K. Hashimoto, A. Fujishima, et al, Nature, 1997, 388, 431). They found that the water contact angle on a TiO2 multi-crystal film could decrease from 72° to 0° after UV irradiation.
This technique has also been applied for anti-fogging coatings (P. K. Sharma, V. S. Veerasamy, 2009, EP 2080740 A1; Z. Zhang, T. Sakakibara, M. Yamada, Y. Kotani, 2003, US2003152763A1). Since this technique is influenced by UV irradiation, an improvement is to combine TiO2 with hydrophilic polymers, or to produce high water retention coating on the substrates. However, TiO2 is relatively expensive. According to ICIS data, the price of TiO2 powder is around $2,535-2,600/ton in Asia by May of 2010, which is an increase of 8.3-15.2% over the price at the beginning of that year. The same cost issue is also surrounded with fluorine based surfactants mentioned earlier.